Ring laser gyroscopes (RLGs) and fiber optic gyroscopes (FOGs) have become widely used technologies in many systems, typically to sense the rotation and angular orientation of various objects, such as aerospace vehicles. Both RLGs and FOGs work by directing light in opposite directions around a closed optical path that encloses an area having a normal along an axis of rotation. If the device is rotated about this axis of rotation, the optical path length for the light traveling in one direction is reduced, while the optical path length for the light traveling in the opposite direction is increased. The change in path length causes a phase shift between the two light waves that is proportional to the rate of rotation.
Generally speaking, the signal to noise sensitivity of such gyroscopes increases as the optical path lengths and diameters of the closed optical path are increased. In this sense, both RLGs and FOGs have an advantage in that light is directed around the axis of rotation multiple times. In RLGs, a series of mirrors is used to repeatedly reflect the light around the axis, forming a high finesse resonator. In FOGs, the light travels around the axis through a coil (with numerous turns) of optical fiber, which often has a length of several kilometers.
In recent years, resonator fiber optic gyroscopes (RFOGs) have been developed which combine the above-described path length benefits of RLGs and FOGs into a single device that uses both a recirculating element, such as a mirror or a fiber coupler, and a multi-turn optical fiber coil to form an optical resonator. The combined benefits of RLGs and FOGs allow RFOGs to use shorter optical fiber and to be relatively small. One potential difficulty associated with RFOGs is that phase shifts can occur that are not attributable to rotation, but rather to the fact that monochromatic light is propagating in a glass medium provided by a conventional optical fiber.
Even more recently, Sagnac interferometers, like those used in FOGs, have been used to sense other phenomena, such as electric current. Typically, the implementation of such sensors, as well as RFOGs, is difficult and expensive. One reason for this is because the conductor through which the current is to flow may either be cut or disconnected from its electrical system, or a transformer may be installed in series with the current. The above-noted difficulty associated with RFOGs for rotation measurement applies to current sensing, in that phase shifts often occur that are not attributable to presence of electric current. These errors may be attributed to the propagation of monochromatic light in a glass medium, as provided by a conventional optical fiber.
Accordingly, it is desirable to provide a fiber optic current sensor with improved performance and reduced costs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.